1. Field of the Invention
This invention relates to a method for recovering metal from by-products deposited on the inside of a reactor chamber during processing (e.g., etching, chemical or physical vapor deposition, etc.) of a substrate in the reactor chamber containing a plasma. More specifically, this invention provides an electrochemical method for recovering metal (e.g., platinum, iridium, etc.) from deposits and/or byproducts formed and collected on an internal structure of a reactor chamber wherein substrates are being etched in a plasma of a processing gas.
2. Description of the Prior Art
It is well known that various magnetically enhanced radio frequency (RF) diodes and triodes have been developed to improve performance of plasma reactors. As mentioned in an article entitled xe2x80x9cDesign of High-Density Plasma Sourcesxe2x80x9d by Lieberman et al from Volume 18 of xe2x80x9cPhysics of Thin Filmsxe2x80x9d, copyright 1994 by Academic Press Inc. of San Diego, Calif., these include by way of example only, the Applied Materials AMT-5000 magnetically enhanced reactive ion etcher and the Microelectronics Center of North Carolina""s split cathode RF magnetron. Magnetically enhanced reaction ion etchers (MERIE) apply a dc magnetic field of 50-100 Gauss (G) parallel to the powered electrode which supports a semiconductor wafer. The dc magnetic field enhances plasma confinement, resulting in a reduced sheath voltage and an increased plasma density when the magnetic field is applied. However, the plasma generated in MERIE systems is strongly nonuniform both radially and azimuthally. It is well known that in order to increase process uniformity, at least azimuthally, the magnetic field is rotated in the plane of the semiconductor wafer at a certain frequency, e.g. 0.5 Hz. While this is an improvement, MERIE systems still do not have the desired uniformity and high density in the generated plasma, which may limit the applicability of MERIE systems to next-generation, sub-micron device fabrication.
The limitations of RF diodes and triodes and their magnetically enhanced variants have led to the development of reactors operating at low pressures with high-efficiency plasma sources. These reactors can generate a higher density plasma and have a common feature in that processing power (e.g. RF power and/or microwave power) is coupled to the plasma across a dielectric window, rather than by direct connection to an electrode in the plasma, such as for an RF diode. Another common feature of these reactors is that the electrode upon which the wafer is placed can be independently driven by a capacitively coupled RF source. Therefore, independent control of the ion/radical fluxes through the source power and the ion bombarding energy through the wafer electrode power is possible.
While the limitations of RF diodes and triodes and their magnetically enhanced variants have motivated the development of high-density plasma reactors with low pressures, high fluxes, and controllable ion energies, these developed high-density plasma reactors have a number of challenges. One challenge is the inability of high-density plasma reactors to achieve the required process uniformity over 200-300 mm wafer diameters. High density sources are typically cylindrical systems with length-to-diameter usually exceeding unity. In such cylindrical systems plasma formation and transport is inherently radially nonuniform.
Another challenge is that the deposition of materials on the dielectric window during etching of semiconductor wafers in a process chamber has necessitated frequent and costly reactor cleaning cycles. This is especially true when metals, such as platinum, copper, aluminum, titanium etc., are etched or deposited in the production of integrated circuit (IC) devices. After a metal layer on a substrate has been etched or deposited for a period of time, the etch or deposit rate on the metal may decrease. The dropping in metal etch or deposit rate is due to the build up of conductive by-products deposited on the dielectric window. Such deposited conductive by-products behave as a Faraday shield to reduce the efficiency of rf energy transmission into the plasma by blocking the rf energy transmission through the dielectric window. Thus, there is no stable power transmission into the plasma processing chamber; and there is no efficient power transfer across dielectric windows over a wide operating range of plasma parameters.
A further challenge is that the materials deposited on the inside of a process chamber from etching a conductive metal layer on a semiconductor wafer include metal emanating from the metal layer being etched. This results in a metal loss which is costly especially when the metal layer being etched includes one of the noble metals, such as platinum, palladium, iridium, rhodium, ruthenium, etc. Thus, not only does the deposition of materials on the inside of a process chamber resulting from etching of a metal layer produce conductive byproducts which reduce the efficiency of rf energy transmission and effect the etch rate of the metal layer being etched, but there is also the concomitant loss of metal that was etched from the metal layer.
Therefore, what is needed and what has been invented is a method for recovering metal from metal by-products deposited on the inside of a reactor chamber during processing of a substrate in a plasma reactor chamber. What is further needed and what has been invented is an electrochemical method for recovering metal from etch byproducts produced, by etching of the metal in a reactor chamber containing a plasma of the processing gas.
The present invention provides a method for recovering a metal, preferably a noble metal, from by-products produced in a plasma processing chamber comprising:
a) recovering from a plasma processing chamber a deposit or residue (e.g., by-products) containing a metal, preferably a noble metal such as platinum or iridium;
b) disposing (e.g., preferably by dissolving) the deposit including the metal in a liquid, such as an acid (e.g., hydrochloric acid and/or nitric acid) and water mixture; and
c) recovering the metal from the liquid.
In a preferred embodiment of the present invention for the immediate foregoing method, the liquid preferably comprises an acid such as hydrochloric acid. In a preferred embodiment of the invention, the liquid comprises an acid (e.g. hydrochloric acid) plus another acid (nitric acid) and/or deionized water. In another preferred embodiment of the invention, the liquid comprises a HNO3:HCl:H2O mixture to dissolve a metal rich deposition, such as a platinum rich deposition in which Pt2+, Pt4+ and metal Pt exist. A HCl:HNO3 mixture having 3:1 mixing ratio by volume has been found acceptable, or a HCl:HNO3:H2O mixture having a 2:1:1 mixing ratio by volume, or a 1:1:1 mixing ratio by volume, has also been found acceptable. In yet another preferred embodiment of the invention, the liquid comprises from about 0% vol. to about 40% vol. deionized water, from about 20% vol. to about 100% vol. hydrochloric acid (e.g. 37% by wt in deionized H2O concentrated hydrochloric acid with 12N concentration) and from about 0% vol. to about 60% vol. nitric acid (e.g. 67% by wt in deionized H2O concentrated nitric acid with 15N concentration). The recovering step (c) may preferably comprise inserting a working electrode into the liquid until the liquid reaches a first level on the working electrode, and passing a current through the working electrode. Subsequently, the working electrode may be further immersed into the liquid until the liquid reaches a second level on the working electrode, and the amount of current flowing through the working electrode may be increased. The second level on the working electrode is preferably higher than the first level on the working electrode such that the working electrode is deeper in the liquid. The immediate foregoing method preferably additionally comprises etching, prior to the recovering step (a), a layer of the metal in the plasma processing chamber to produce the deposit containing the metal. The current passing through the working electrode preferably ranges from about 0.2 amps to about 5.0 amps, and the increased current flow is such that total current flow preferably ranges from about 0.4 amps to about 10.0 amps; thus, the amount of current increase ranges from about 0.2 amps to about 5.0 amps. Beyond 2.0 amps, EGUG large current potentiostat can be used.
In a preferred embodiment of the present invention, the recovering step (c) in the above method may also preferably comprise depositing at a first rate the metal on a working electrode, and subsequently depositing at a second rate the metal on the working electrode with the second rate being higher than the first rate. The recovering step (c) includes removing the working electrode from the liquid, and removing the metal from the working electrode. Prior to removing the working electrode from the liquid, the recovering step (c) in the above method may further also preferably comprise selecting and maintaining a desired change in potential of a reference electrode with respect to a working electrode by causing a current to flow, in response to the selected desired change in potential, through the working electrode and through a counter electrode of a magnitude sufficient to effect the selected desired change in potential of the reference electrode with respect to the working electrode and cause the metal to be removed from the liquid and deposit on the working electrode. The magnitude of the current flowing through the working electrode and the counter electrode may be measured, as well as the difference in potential between the working electrode and the reference electrode.
According to standard electrode potentials in aqueous solutions at 25xc2x0 C. in V vs. standard hydrogen electrode, the following reactions exist:
Since the actual electrolyte has higher acid concentration, the standard reduction potential of the solution after dissolving the deposition is +0.802V vs. SCE. The solution contains Pt2+ and Pt4+ as complex ions PtCl42+ and PtCl62+ after applying negative (cathodic) potentials in a range of xe2x88x920.50V to xe2x88x921.2V vs. SCE. Actually the applied potential difference vs. solution equilibrium potential +0.802 is at 1.302V to 2.002V. The larger the cathodic potential applied, the larger the cathodic current, and the higher the Pt deposition rate is.
The present invention also provides a method for recovering a metal, preferably a noble metal, from by-products produced in a plasma processing chamber comprising the steps of:
a) recovering from a plasma processing chamber a deposit (e.g., by-product residue) containing a metal, preferably a noble metal such as platinum or iridium;
b) disposing the deposit including the metal in a liquid;
c) inserting into the liquid of step (b) a first electrode, a second electrode and a third electrode;
d) applying a difference in potential (e.g., from about xe2x88x920.5 volts to about xe2x88x921.2 volts vs. saturated calomel electrode) between the first electrode and the second electrode to cause metal to deposit on the first electrode; and
e) removing the metal from the first electrode.
In the immediate foregoing method, the disposing step (b) preferably comprises dissolving the deposit in the liquid, which comprises at least one acid and a pH of less than about 7.0 (e.g. preferably about 1.0 or less). The liquid additionally preferably comprises deionized water, and the at least one acid comprises hydrochloric acid and/or nitric acid. More specifically, the liquid preferably comprises from about 0% vol. to about 40% vol. deionized water, from about 20% vol. to about 100% vol. hydrochloric acid, and from about 0% vol. to about 60% vol. nitric acid. As previously indicated, the hydrochlorine and nitric acid typically are diluted with deionized water such that HCl is 37% by wt. in deionized H2O with a 12N and HNO3 is 67% by wt. in deionized H2O with a 15N. The inserting step (c) preferably additionally comprises inserting the first electrode into the liquid until the liquid reaches a first level on the first electrode, and the applying step (d) causes a current to pass through the first electrode. This causes the metal to deposit and/or become adhered to the first electrode at a first rate.
The inserting step (c) also preferably additionally comprises inserting the first electrode into the liquid until the liquid reaches a second level on the first electrode, and increasing the amount of current flowing through the first electrode. This causes the metal to deposit and/or become adhered to the first electrode at a second rate, which preferably is greater than the first rate. The second level on the first electrode may be higher on the first electrode than the first level on the first electrode such that the first electrode is deeper in the liquid. The deposit is preferably obtained by etching a layer containing the metal in the plasma processing chamber. The method additionally preferably comprises increasing the difference in potential between the first electrode and the second electrode to cause the metal to be deposited on the first electrode at a second rate which is higher than said first rate. The increased difference in potential preferably ranges from about xe2x88x920.5 volts to about xe2x88x921.2 volts.
Another embodiment of the present invention provides an assembly for recovering a metal, preferably a noble metal, from by-products produced in a plasma processing chamber comprising a liquid, and by-products containing a metal, preferably a noble metal, removed from a plasma processing chamber and disposed in the liquid. An electrode assembly is also disposed in the liquid and includes a first electrode, a second electrode and a third electrode. A potential change selecting means (e.g., a potentiostat or the like) is electrically connected to the first electrode, to the second electrode, and to the third electrode for selecting and maintaining a desired change in potential of the second electrode with respect to the first electrode by causing a current to flow, in response to the selected desired change in potential, through the first electrode and the third electrode of a magnitude sufficient to effect the selected desired change in potential of the second electrode with respect to the first electrode, and cause the metal to be removed from the liquid and deposit on the first electrode. The assembly of this embodiment of the present invention additionally comprises a means (e.g., an ampmeter), electrically engaged to the first electrode, for measuring the magnitude of the current flowing through the first electrode. Preferably, the first electrode is a working electrode, the second electrode is a reference electrode, and the third electrode is counter electrode. The liquid comprises at least one acid.
Another embodiment of the present invention provides an assembly for recovering a metal, preferably a noble metal, from by-products produced in a plasma processing chamber comprising a liquid, and by-products containing a metal, preferably a noble metal, removed from a plasma processing chamber. An electrode assembly is also disposed in the liquid and includes a first electrode, a second electrode and a third electrode. A current selecting means (e.g. a potentiostat or the like) is electrically connected to the first electrode, to the second electrode, and to the third electrode for selecting and maintaining a desired current flow through the second electrode and the third electrode, and cause the metal to be removed from the liquid and deposit on the first electrode. The assembly of the present invention additionally comprises a difference in potential measuring means (e.g., a voltmeter) for measuring the magnitude of a difference in potential between the second electrode and the first electrode.
The foregoing provisions along with various ancillary provisions and features which will become apparent to those skilled in the art as the following description proceeds, are attained by this novel assembly and method, a preferred embodiment thereof shown with reference to the accompanying drawings, by way of example only, wherein: